Thin film transistor liquid crystal display (TFT LCD) technology has been developed as a successor to cathode ray tube technology for color display computer terminals. Liquid crystal displays with a given display screen area occupy a smaller volume than cathode ray tube devices with the same screen area. This is considered commercially significant, because the smaller liquid crystal display has a smaller footprint. That is, it occupies less area on a user's desk or terminal stand. Liquid crystal display devices may also have lower power requirements than corresponding cathode ray tube devices. This is because the operating voltages of liquid crystal devices are considerably lower than the operating voltages of cathode ray tube devices. These characteristics are not only desirable for computer terminals but are also desirable for televisions, video displays, and a variety of other electronic devices.
While different types of color liquid crystal display devices exist, such devices generally include two spaced panels which define a sealed cavity filled with a liquid crystal display material. A transparent common electrode is formed inside the defined cavity on one of the glass panels. Individual electrodes, also on the inside of the defined cavity, are formed on the other glass panel. Each of the individual electrodes has a surface area corresponding to the area of one, or part of one, picture element or pel. Each pel is too small to be easily seen by the unaided human eye. If the device is to have color capabilities, it must also include color filters with red, green, and blue color areas. Each color area is aligned with one of the electrodes. Each set of red, green, and blue color areas is grouped into a triad, repetitive stripes, or other consistent pattern within the pel.
In typical liquid crystal displays (LCDs), each of the individual electrodes can be addressed by means of a thin film transistor. Depending upon the image to be displayed, one or more of the pel electrodes is energized during the display operation to allow full light, no light, or partial light to be transmitted through the color filter area associated with the pel electrode. The image perceived by the user is a blending of colors formed by the transmission of light through adjacent color filter areas.
The display may be backlighted by locating a light source on the opposite side of the display, away from the viewer. Alternatively, the display may include a reflective layer at its rear surface and rely on the light source located on the same side of the display as the viewer.
Color filters for use on such devices have been fabricated using a number of different approaches. One approach has been to spin or deposit a light sensitized adhesive film onto the glass panel. The film is then patterned in three sequential steps. During each step, dye of a specific color is applied to the predetermined regions of the film. According to another approach, organic pigments are deposited by vacuum evaporation. These pigments are then patterned by conventional lift-off techniques. According to still another approach, a dyed and patterned stretched film material is used to create an internal color polarizing filter.
Each of these approaches has certain drawbacks. Most involve polymer deposition and photopatterning techniques which are relatively expensive and difficult to perform with the necessary precision. This is especially true when the devices are manufactured on a volume basis. Also, each creates a color filter film which is located between the transparent common electrode and the individually addressable pel electrodes (referred to as the gap of the device). To increase optical density (or color intensity) of color filters made by the above described approaches, the thickness of the color filter film may be increased. However, increasing the filter film thickness also increases the spacing between the transparent common electrode and the individually addressable pel electrodes. The increased spacing causes the gap and associated LCD thickness to be more nonuniform which results in nonuniform electrical characteristics of the LCD.
One of the major problems with the approaches described above is the difficulty of maintaining registration or alignment between the pel electrodes and the color regions in the color filter layer. The prior art color filters are composed of organic materials which require processing on the glass panel itself. The organic materials are not stable at high temperatures. This means that the Thin Film Transistors (TFT) must be formed prior to, or separate from, the color film deposition. Misregistration problems occur in either case, because when the two films are made separately and brought together, the error in joining the transistors to the filters may be large compared to the pel size. Also, when the TFT is made before the color film, the color film has to be printed and dyed on the pel so that the color film position compensates for the TFT activated light transmission. Forming the TFT before the color filter or forming it separately from the color filter exacerbates already difficult registration problems because of the organic nature of the filter.
Still another approach to making TFT LCDs uses photosensitive emulsion layers. A liquid crystal shutter device is used to sequentially expose predetermined areas of the photosensitive emulsion layers while the layers are flooded with light having the specific color. Three differently colored regions are formed by sequentially energizing three different groups of pel electrodes through associated thin film transistors. The latent images produced in the photosensitive emulsion layer are developed and the film is laminated to a glass substrate to form a multicolored filter.
While the use of photosensitive emulsion layers simplifies registration or alignment problems, it nevertheless retains the drawbacks of some of the other approaches discussed earlier. Multiple exposure operations to different colors of light are required along with a step of laminating photosensitive emulsion layers to a glass substrate. The number of steps and the relative complexity of the those steps necessarily must be reflected in the product cost.